If you got a copy of the OS on CD, then it is just a matter of making a copy. If the manufacturer provided a Rescue/Recovery disk containing the OS you can make a copy of that instead.
If you have neither, you cannot simply make a bootable copy of the OS, but you can make a backup image of your system by using Programs/Accessories/System/backup and selecting a full backup. This will burn an image of your system to CD and produce a system recovery boot disk (floppy).
Regardless, make sure you have a copy of the OS registration number as you will need it to reinstall. Replacements are not provided.